1. Field of the Invention
This invention relates to composite metallic articles formed by treating the surfaces of a plurality of segments to alter their physical properties and joining the segments, alone or with other treated segments, into a single composite body.
2. Prior Art
A variety of surface treatments and coatings have been developed to provide parts formed of metal with surface properties which are metallurgically incompatible with the necessary bulk properties of the part, and/or too expensive to achieve throughout the part. For example, the cam shaft of a reciprocating engine must be relatively ductile so as to withstand the bending stresses imposed on it by the unbalanced weights of its lobes yet its cam surfaces must be very hard to withstand the friction forces imposed by their sliding contact with the valve lifters without excessive wear. Since no metal which might be practically used as a cam shaft combines the necessary ductility and wear resistance, cam shafts are typically formed by employing a ductile steel shaft and hardening the surface of the shaft at points contacted by the valve lifters with a carburization process. While improving surface hardness is the most common reason for the application of surface coatings and treatments, these techniques may be used for other purposes such as providing a relatively soft bearing surface on a hard steel shaft.
An inherent disadvantage of these coated or surface treated metal parts is their limited life resulting from inevitable wear of the surface coatings. The coatings are limited in thickness because of the nature of the metallurgical processes involved or because of the inherent physical limitations of the coating. For example, surface hardening by carburization is typically limited to a depth of about 1/8 inch. Coatings consisting of a grit of tungsten particles suspended in a softer metal matrix are difficult to apply in depths of more than about 1/2 inch in typical applications.
Many of these coating and surface treatment processes are also severely limited in the configuration of the surface to which they may be applied. For example, certain of the coating techniques can only be applied to a relatively flat surface while it is disposed in a horizontal attitude with unlimited clearance from above.